Light responsive device



p 1936- F. H, SHEPARD, JR

LIGHT RESPONSIVE DEVICE Fi led March 8, 1955 INVENTOR. FRANISH.SHEPAP\DJR.

Patented Sept. 22, 1936 LIGHT RESPONSIVE DEVICE Francis H. Shepard, Jr.,Rutherford, N. J., assignor, by mesne assignments, to Radio Corporationof America, New York, N. Y., a corporation of Delaware Application March8, 1935, Serial No. 9,932

3 Claims.

My invention relates to improvements in phototube photometers. I

The usual photometer comprises a resistance, a battery and a phototubeconnected in series. The intensity of light exposed to the phototubedetermines the magnitude of the current flow in this series circuit. Anelectric meter is connected to the resistance to-measure the currentflowing thru it or the voltage drop across it and is so calibrated as togive a direct reading of the intensity of light upon the phototube. Inoperation the meter is calibrated by noting the needle deflection whenthe phototube is exposed to sources of light of known intensities.Thereafter unknown light intensities may be measured by reading theelectric meter provided the electrical constants of the photometerremain unchanged. It has been found in practice, however, that such anarrangement of comparing light sources is subject to error because, forexample, the resistance between the terminals of the phototube maychange. This change inresistance would obviously change the current flowin said series circuit', which would destroy the calibration of themeten More important, the leakage current thru the phototube causes acurrent flow in the phototube loading impedance even with zero light.Changes in. leakage resistance, then, cause changes in the normal. orzero setting of thephototube which precludes standardization of thephototube current values in terms oilight values. This change in theresistance is due to'several factors, such as dirt on the bulb of thephototube,- changes in humidity'and other factors which affeet surfaceleakage paths over the interior and exterior walls of the tube, as wellas variations in leakage paths thru condensers and other elementsconnected acrossthe phototube.

An object of myinventi'on, therefore, is to pro duce a photometer withmeans to compensate for changes in static conditions or parameters inthe photometer.

Another object of my invention is to construct a light responsivedevice, the current values of which are easy to calibrate in terms oflight values, which is rugged in construction and which is stable inoperation; v

I accomplish the results desired'by energizing the photometer withalternating current as distinguished from the usual batteries, and byconnecting an adjustable resistance of high value in shunt across theterminals of the phototube. The voltage drop across this adjustableshunt resistor is impressed upon a suitable recording relay. By suchconnections the adjustable resistor is placed in parallel relation withthe resistance leakage paths between the terminals of the phototube, theleakage resistance and the adjustable resistor constituting a parallelresistance load across the phototube. If'the leakage resistance variesafter calibration of the photometer with a finite value of loadresistance across the phototube, this load resistance may thereafter beadjusted to said finite value simply by manipulating the adjustableresistor; for example, if the leakage resistance between the terminalsof the phototube should be reduced by moisture upon the base or envelopeof the phototube or byJeakage in the associated circuits the shuntadjustable resistor would be increased in value. So that maximumfeasible impedance between the terminals of the phototube may bemaintained, with the attendant high degree of phototube sensitivity, analternating current source is employed to energize the phototube and isconnected to the phototube thru a coupling means such as a condenserwhich ofiers high impedance to direct current. When alternating currentsupply is employed to energize my improved photometer, the rectifyingaction of the phototube serves to convert the al ternating current todirect current in'the load impedance. That is, the current rectified bythe phototube produces across the adjustable resistor a direct currentvoltage which actuates any suitable recording relay or electric meter.

A better understanding of my invention may be had by referring to theaccompanying drawing in which Figure 1 shows a light responsive circuitembodying my invention; and Figure 2 shows a modification thereof inwhich the phototube loading resistor and alternating current supply isslightly varied in connections.

Referring to Figure 1, in the circuit of my device any desired form ofphototube may be employed, as for example the one shown in which theenvelope l encloses an anode 2 and a light activated cathode electrode3. For the purpose of energizing the phototube an alternating currentsupply, which may be connected to the conventional 60 cycle powersource, is connected across the terminals of the phototubes in themanner shown. To maintain high impedance between the terminals of thephototube, in one. lead between the winding 4 and the connectedelectrode is inserted a condenser 5. Condenser 5 is preferably soproportioned as to present low impedance to the impressed alternatingcurrent and high impedance to direct currents. A load resistor 6 of highvalue, for example of the order of several megohms, is connecteddirectly across the output of the phototube. For reasons which willappear hereinafter, resistor 6 is made adjustable. At 1 is shown athermionic relay with its input electrodes 8 and 9 connected acrossloading impedance 6 thru a filter, which by way of example is shown as aresistor Ill and a condenser ll. Elements l and II of the filter are soproportioned as to smooth out the fluctuations in voltage appearingacross impedance 6. With the exposure of light upon the cathode 3, aspace current is caused to fiow between the anode 2 and cathode duringthe impression of each positive loop of the alternating voltage upon theanode. By virtue of the rectifying action of the phototube a directcurrent potential is built up across the impedance 6, the amplitude ofwhich is controlled by the conductivity of the phototube. Conductivityof the phototube is in turn controlled by, and is directly proportionalto the intensity of light impressed upon the cathode. It is seen,therefore, that the intensity of light upon the phototube controls thedirect current potential upon the grid 8 of the thermionic relay. Theoutput of the relay may be connected thru a milliameter or any desiredrecording instrument calibrated in terms of lumens or light intensity.

In practice it has been found that an appreciable reduction in voltagemay be caused between the terminals of the phototube by current leakagepaths over and thru the insulation of the envelope and base members ofthe phototube and thru the insulation of the connected condensers. Theseleakage paths are represented diagrammatically at I2, 13 and [4.Resistance l2, further, is found to vary substantially, due to severalfactors, such as changes in humidity, and moisture and shifting depositsof dirt on the bulb, and other factors which affect the surface leak agepaths over the interior and exterior walls of the bulb and base. Theleakage paths thru condensers and I I, which are essentially in parallelwith resistance I2, are similarly afiected. By reason of the instabilityof these resistances between the terminals of the phototube it isdifiicult to maintain the calibration of the circuit. For example, ifthe output of relay 1 is recorded for a given light intensity upon thephototube and for a finite value of load impedance across the terminalsof the tube, any change thereafter in,

load impedance would result in a corresponding change in voltage acrossimpedance 6 and a corresponding change in anode current thru relay 1. Asa convenient means for compensating for these changes in leakageresistances l2, l3 and 14, load resistor 6 is made adjustable. Sinceresistances 6, l2, l3 and M are in parallel, resistance 6 may at anytime be manipulated to bring the total of the parallel resistance to anyfinite value which conveniently may be of a value corresponding to theresistance employed when calibrating the phototube and its circuits.

.As' distinguished from the usual practice in which the phototube isenergized by a battery or other source of potential of low impedanceconnected across the terminals of the phototube, I employ an alternatingcurrent source 4 isolated as to direct current from the phototube bycondenser 5. Because of the high direct current impedance of the sourcesubstantially the full supply voltage is maintained across thephototube.

In Figure 2 is shown my invention with a modified form of power supplyconnections. Here the alternating current supply winding 4 is connectedin series with the phototube thru condenser II and as in Figure 1resistances 6, I2 and I4 are connected for direct current in paralleland across the terminals of the phototube. The rectifying action ofphototube I produces a direct current potential across the ends ofloading impedance 6. The control circuit of relay 1 is connecteddirectly across impedance 6. In this modification condenser is found toprovide sufficient filtering action for the pulsating voltages acrossimpedance 6. Here, as in Figure 1, impedance 6 may be adjusted tocompensate for any unwanted variations in leakage resistance H thusproviding a light measuring device which can be readily adjusted forchanges in static parameters.

In my improved circuits, it is significant that although variations inleakage resistance across the phototube cause changes in sensitivity ofthe phototube, the leakage resistance changes cannot affect the zerolight value of current in the output of relay 1. That is, with zerolight and zero phototube space current, the direct current potentialacross coupling impedance 6 is zero.

7 While I have shown the preferred embodiments of my invention it isobvious that many variations may be used within the scope of myinvention. It is accordingly desired that my invention be limited onlyby the prior art and by the accompanying claims.

I claim:

1. In a light measuring circuit, a phototube comprising an envelopecontaining an anode electrode and. a light irradiated cathode electrode,and terminal members in the wall of said envelope for said electrodes, aload impedance for said phototube comprising a resistance having a.value of the order of the leakage resistance between said terminalmembers connected at one end to one of said terminal members, andconnected at its other end to shunt the resistance between the terminalsof said phototube said re sistance being adjustable to adjust the totalinter-terminal impedance to a predetermined value, and means forapplying an alternating'voltage across the terminals of said phototube.

2. In a photometer, a phototube comprising an anode and a lightresponsive cathode, an adjustable resistor of relatively high valueconnected directly between said anode and said cathode, a circuit ofhigh direct current impedance and low alternating current impedanceconnected between said anode and said cathode, and a source ofalternating current potential in said circuit, and the input of a relayconnected across said resistor.

3. In a light responsive device, a phototube comprising an anodeelectrode and a cathode electrode, an alternating current source inseries with a condenser connected between said electrodes, a loadresistor connected directly between said electrodes, an electron relaywith an input circuit, and means for coupling said input circuit to saidresistor.

FRANCIS H. SHEPARD, JR.

